Magnetron having spurious signal suppression means

ABSTRACT

In a magnetron, at the end of the cathode terminal thereof is provided a noise-eliminating filter comprising at least two inner conductors of microstrip type transmission line, disposed substantially parallel to and near each other, and an outer conductor partially wrapping the inner conductors, with microwave attenuating substance filled between the inner conductors and between the inner conductors and the outer conductor.

United States Patent Harada et al. 1 Jan. 7, 1975 [541 MAGNETRON HAVING SPURIOUS SIGNAL 3,458,755 7/1969 Staats 315/3951 SUPPRESSION MEANS 3,543,082 11/1970 Boehm 315/3951 X 3,551,858 12/1970 Cielo 333/79 1 Inventors: Aklkalu Harada; HIrOShI HlSada, 3,573,673 4/1971 De Vries 333/72 both Of Mobara, Japan 3,697,804 10/1972 Anderson et a1. 315/3951 3,727,098 4/1973 Crapuchettes 315/3951 [73] Asslgme- Ltd-1 Tokyo Japan 3,732,459 5/1973 Oguro et a1 315 3951 [22] Filed: Aug. 28, 1973 Primary ExaminerJames W. Lawrence [21] Appl. No.: 392,299 Assistant Examiner-Saxfield Chatmon, Jr.

Attorney, Agent, or Firm -Craig & Antonelli [30] Foreign Application Priority Data Sept. 1, 1972 Japan 47-87171 1571 ABSTRACT In a magnetron, at the end of the cathode terminal [52] 11.8. C1 315/3951, 315/3953, 333/79, thereof is provided a noise-eliminating filter compris- 333/84 M ing at least two inner conductors of microstrip type [51] Int. Cl. H0lj 25/50 transmission line, disposed substantially parallel to and [58] Field of Search 315/3951, 39.53; 333/72, near each other, and an outer conductor partially 333/79, 84 M wrapping the inner conductors, with microwave attenuating substance filled between the inner conductors [56] References Cited and between the inner conductors and the outer con- UNITED STATES PATENTS ductor- 3,020,447 2/ 1962 Schall et a1. 315/3951 10 Claims, 9 Drawing Figures PATENIEUJAH 71975 3859558.

SHEET 10F 4 Fl 6.! PRIOR ART Fl PRIOR ART PATENTEDJAN W5 3.859.558

SHEET 2 OF 4 26 Illlllllllhlx 24b FIG 4b FIG. 4u

27 l i M22223 L 1 FIG 4C The present invention relates to the structure of a magnetron used especially for a microwave oven, and more particularly to the structure of a spurious signal suppression means or noise eliminating filter for use with such a magnetron.

For better understanding of the prior art and the present invention the following specification should be read with the aid of the attached drawings, in which:

FIG. 1 shows an example of a conventional magne= tron in its front view, partially broken;

FIG. 2 is a cross section taken along the line II-II in FIG. 1;

FIG. 3 shows an example of a filter used with a magnetron accordingto the present invention, in the perspective view, partially broken;

FIGS. 4a to 4c show in plan views examples of a microstrip type transmission line and a meander line which are used as filter elements and constitute the filter as shown in FIG. 3;

FIG. 5 shows an embodiment of a magnetron according to the present invention, in the front view, partially broken;

FIG. 6 is a cross section taken along the line VI-VI in FIG. 5; and

FIG. 7 illustrates the attenuation characteristics.

In a magnetron a wave of fundamental frequency, its

harmonics and waves of VHF and UHF, are fed out of the cathode circuit during oscillation. It is therefore necessary to provide the magnetron with a noiseeliminating filter (hereinafter referred to simply as fil- Such a filter usually takes the form of a combination of an inductor and a capacitor, connectedwith the";

cathode terminal of the magnetron. The conventional way of providing such a filter will be described in detail with the aid of the drawings. a

FIGS. 1 and 2 show in partially broken front view and cross section a conventional magnetron witha filter,'

the parts except principal ones being omitted. In F IG.

for heat dissipation with cooling fins 2 made of aluminum or copper. A permanent magnet 3 supplies magnetic flux through yokes 4 and4a into the operating space (not shown) in the center of the anode 1. An antenna 5, supported by an insulating cylinder 6 of for example glass, serves to emit into an oven such as a miof the insulator 9 which is not fixedito the yoke 4a are fixed by means of a screw 11c a tefininal lug 12a pressure-connected with the cathode lead 8 and a terminal lug 12b pressure-connected with one end of an inductor 13 consisting ofa insulator-coated copper wire solenoid coil and a ferrite rod-(not shown) inserted in the coil to absorb noises. On that end of the capacitor 10 which is not fixed to the'yoke 4a are fixed by means of a screw 11d a terminal'l'ug 12c pressure-connected with the other end of the. inductor 13 and a terminal lug 12d which is pres sure connected with one end of a cathode lead-out "'wire 14 whose other end is pressurecorinected with a terminal lug 12e. The device according to the present invention has such cathode leads 8, insulators 9, capacitors l0, inductors 13, and cathode lead-out wires 14, all in pairs, as shown in FIG. 2. The inductor 13 and the capacitor 10 constitute a filter. In contact with the cooling fins 2 is provided a shield case 15 of metal such as aluminum, in the form of a hollow cylinder. The shield case 15 is cramped onto the fins 2 by means of a metal belt 16 and these three members are fixed to each other through spot welding etc. The shield case 15 serves also as a wind-cooling duct. A lid 17 to close the top end 18 of the shield case 15 is fixed to the case 15 by means of a self-tapping screw 19 after the attachment of the filters has been completed. A multiplicity of perforations 20 are cut through the main surface 17a of the lid 17 to allow the air for forced cooling to be fed toward the fins 2. The cathode leadout wires 14 are taken out of the case 15 through perforations 15c cut in the shield case'l5.

7 ,f With this structure, in which the filters are constit'uted of inductors 13 and capacitors 10 of lumped constant type, the filtering effect is satisfactory for noises at VHF and below, but the filters lose their function considerably at UHF and microwave frequencies. "Namely, the inductor 13 becomes capacitive at extremely high frequencies, due to its distributed capaci- .loses its electrostatic capacitance with the increase in 1 an anode 1 having a well-known structure is provided frequency. For these reasons, the capacitor 10 cannot serve as a stable capacitance element, either. The noise waves propagating through the cathode leads 8 have crowave oven high frequency electromagnetic waves generated in a cavity resonator (not shown) defined in the anode l, to heat raw food to be cooked. A cathode (not shown), which is disposed near the center of the anode 1 to form the operating space together with the anode 1 and serves to supply electrons necessary to maintain the high frequency oscillation, is rigidly supported on and electrically connected with a plurality of cathode support bars not shown in the figurefThose ends of the cathode support bars which are not concurrent does, and the other in which current flows parallel through the cathode leads 8 and returns through ground. In the former mode, the LC filters are arranged in series so that the effect of attenuation is additive.

while in the latter inode the filters are parallel to each other so that the amount of attenuation'is halved.

I Moreover, spurious waves due to the radiation from the cathode leads8 and the inductors 13 are received by the cathode lead-out wires 14 and therefore with the. I

increase in frequency the noise attenuation characteristics ofthe filters are deteriorated and also the frequency characteristics become extremely nonuniform. Especially, at 2.45 GHz, which is the fundamental frequency of a magnetron used for a microwave oven the wave leakage from the cathode lead 8 is considerable so that such a conventional filter as having a small amount of attenuation is not satisfactory from the standpoint of safety. Moreover, the leakage of the fun- J 3 damental waves adversely affects the quality of the picture of UHF telg ision systems.

Further, in order to operate the conventional magnetron a voltage tfrom the commercial supply is applied to the terminal. ,l'ugs 12e of the cathode lead-out wires 14 so as to heafthe cathode filament while a high negative voltage,(with respect to the anode) is empressed between ihe terminal lugs 12e so as to excite the electrons into o scillation. In this case, the path between the cathodei lad-out wires 14, through which a rather heavy current to heat the cathode filament flows, consists of zimixltiplicity of parts and therefore junction points, say about ten such points, so that the reliability of these tjunctions is lowered, and that the multiple parts need a plurality of different fabrication steps.

It is therefore one object of the present invention to quency noise waves is effectively prevented.

Another object of the present invention is to provide a magnetron which is simple in construction and can be easily fabricated.

A further object of the present invention is to provide a magnetron which can be produced at lower cost.

An additional object of the present invention is to provide afilter for use with a magnetron, which is easily fabricated, has a low production cost and has an excellent attenuation characteristic for high frequency noise signals. 7

' The feature of the present invention is the use ofa filter consisting of microstrip type transmission line of a filter element and a microwave attenuating substance.

According to the present invention is proposed a magnetron comprising an anode; a cathode defining an operating space together with the anode; a plurality of cathode support bars to support the cathode; cathode leads connected with the plural cathode support bars and cathode lead-out wires connected with an external source, to supply electric power from the source for the cathode; a filter for eliminating noise signals, connected between the cathode leads and the cathode lead-out wires; a stem to connect the cathode support bars with the cathode leads in vacuum condition; and a shield case to contain therein the filter and the stem, wherein the filter consists of at least two microstrip type transmission lines for choking at least one predetermined frequency component of an electromagnetic wave propagating therethrough and microwave attenuating substance for attenuating the wave over a wide frequency range.

Now, the present invention will be described by way of example with the aid of the attached drawings.

FIG. 3 shows in perspective view, partially broken, an embodiment of a filter for use with a magnetron, according to the present invention. In FIG. 3 is shown a filter 21 consisting of a pair of inner conductors 22 and 23 disposed near andparallel to each other and of an insulating microwave attenuating material (hereafter referred to as microwave absorbing layer") of, for example, rubber mixed with ferrite powder or sintered ferrite plate. These inner conductors are made of good conductor such as copper or aluminum and have a form of strip whose cross sectional area is large enough to withstand a current of to amperes for heating the cathode, and each inner conductor is formed into a predetermined pattern so as to provide filtering characteristic as described later. The inner conductors having such a pattern may be obtained by the punchingthrough of a conductor plate having a suitable thickness. The microwave absorbing layers 24a, 24b and 240 serve to rigidly support the inner conductors 22 and 23 at a constant distance from each other and also to cover them except their end portion. An outer conductor 25 of conductive material envelops the microwave absorbing layers 24 except their portions where the inner conductors 22 and 23 are drawn out. The outer conductor 25 may be made ofa thin metal sheet, plated film or vapor-deposited film since it is free from the heater current. An insulating layer 26 of a material having a high breakdown voltage may be inserted between the microwave absorbing layers 24 and the outer conductor 25 so as to increase the withstand voltage between the inner and outer conductors if the insulation against a voltage of several thousand volts applied usually between the inner and outer conductors cannot be assured solely by the microwave absorbing layers 24.

The inner conductors 22 and 23 shown in FIG. 3 have such patterns as shown in FIGS. 4a and 4b. Each inner conductor may have one of the patterns or may have a meander line (flat crenellation transmission line) as shown in FIG. 40 inserted in a portion of the transmission line having one of the patterns along transmission direction. The theory on the microstrip type transmission line teaches that there is a relationship between the length l of the projection 27 or 27a of the inner conductor shown in FIG. 4a or 4b and the wavelength A of the electromagnetic wave in free space for which the amount of attenuation becomes maximum, such that I A /4 m where e and p. are the mean dielectric constant and magnetic permeability of the medium surrounding the transmission line in question. In the embodiment shown in FIG. 3, the medium consists of the microwave absorbing layer 24a, 24b and 24c of ferrite material. The noise signals generated by the magnetron have high electric field intensity specifically at the fundamental frequency, second harmonic and third harmonic but not over the entire frequency band. Accordingly, the values of] are calculated from the above equation by substituting the'values of the frequencies corresponding to the fundamental oscillation, second harmonic and third harmonic for A in the equation. The projections 27 and 27a (usually a plurality of such projections 27, 27a, are arranged in series as shown in FIG. 3, so that there is obtained a selective attenuation characteristic having peaks of attenuations corresponding to the frequencies especially desired to be attenuated. The inventor used in thier experiments microwave absorbing layers of ferrite material having the means valuesofe and t such that e 3 and p. 20. In this case, the value of I for which the fundamental frequency of 2.45 GHz of a magnetron for a microwave oven is specifically attenuated, is about 4 cm while the values for the second and third harmonics are respectively about 2 cm and 1.3 cm. Therefore, if the inner conductors 22 and 23 are provided with projections having suitable values of l, a noise eliminating filter can be obtained which is well adapted for a magnetron used in a microwave oven. It should here be noted that the value of! calculated from the above equation depends on the oscillation frequency corresponding to the purpose of application of the magnetron and to the associated material used as the microwave absorbing layers.

The meander line as shown in FIG. 4c serves to increase the effective length of interaction between the microwave abosrbing layers and the surface waves, and provides an attenuation characteristic over a broader frequency band. It should also be noted that the shape of the projection provided in the surface wave transmission line is not limited to a rectangular one as shown in FIG. 4a and FIG. 4b but may be modified variously, e.g., elliptic, as is apparent from the general theory on filters.

FIGS. 5 and 6 are respectively a partially broken front view and a cross sectional view of a magnetron embodying the present invention, provided with a filter having such a structure as described above, these figures corresponding respectively to FIGS. 1 and 2. The same reference numerals and characters are applied to like parts or elements as in FIGS. 1 and 2, and the description of the like constituents is omitted. In FIGS. 5 and 6, a filter 21, which has a structure as shown in FIG. 3, has inner conductors 22 and 23 whose extensions 22a and 23a are bent so as to be inserted into a pipe connector 30 fixed to an insulating cylindrical stem 7 and to be pressure-connected there. The pipe connector 30 is electrically connected in the stem 7 with cathode support bars not shown in these figures. The ends of the other extensions 22b and 23b of the inner conductors 22 and 23 are formed into integral terminal lugs 22c and 23c to which an external power source is coupled. The outer conductor 25 of the filter 21 is fixed to and electrically connected with the side wall of a shield case 15, through a crater-like opening a in the side wall of the shield case 15. The opening 15a is so formed as to have a good contact at high frequencies with the outer conductor 25.

In the magnetron having such a structure as described above, the microwave absorbing characteristic due to the ferrite is superposed on the attenuation characteristic of the surface wave transmission line type of filter and a very effective attenuation characteristic can be obtained by determining the lengths [of the projections 27 and 270 corresponding to the frequency spectra of noise signals, so that undesirable leakage of microwaves from the shield case hardly takes place.

FIG. 7 illustrates the results of the comparative measurements of attenuation characteristics exhibited respectively by a filter according to the present invention and a conventional filter, the solid curve A corresponding to the present invention and the dotted curve B to the prior art. In the embodiment of the present invention which is involved to obtain the characteristic curve A, transmission lines having such a shape as shown in FIG. 4b are used as inner conductors 22 and 23, and the values of! for the projections are tuned to the fundamental frequency of 2.45 GHz and the third harmonic of 7.35 GHz which have the greatest tendency of leaking into the cathode circuit. According to the present invention, as depicted by the solid curve A in FIG. 7, an especially large amount, i.e., peak, of attenuation occurs on the flat portion of the curve A over a wide range of frequencies, attributable to the absorption loss of the ferrite, at the fundamental frequency and the third harmonic, so that the proposed filter is highly suited to prevent noise signals from the magnetron since it can almost completely cut off the higher harmonic noises inevitable with the conventional filters. The reason why the peak attenuations occur is as follows. Through the filter according to the present invention are considerably attenuated noise waves having modes in which the waves propagate parallel through the two cathode leads and return through ground since microwave attenuating material such as ferrite is filled between the inner conductors 22 and 23 and the outer conductor 25. Moreover, according to the filter embodying the present invention, having the structure as described above, the cathode leads which tend to radiate noise waves can be made short enough and the filter itself, shielded by the ferrite layers and the outer conductor, radiates no electromagnetic wave. Accordingly, the leakage of microwaves through, for example, the vent holes of the shield case can be reduced so that the radiation of the noise waves from the magnetron as a whole can be reduced.

The dotted curve B shows the relationship between the amount of attenuation and the frequency, exhibited by such conventional filters as shown in FIGS. 1 and 2 in which L SILH and C 500 pF for frequency of l KHz. As seen from the characteristic curve B, the conventional filter'can no longer perform a proper filtering function at and above UHF and the filter has no effect at some specific frequencies.

The inner conductors as constituents of a filter according to the present invention are in the form of a strip and lend themselves readily to mass production using press-stamping. Moreover, according to the present invention, there is no need for expensive filter elements such as a barium titanate ceramic capacitor whose breakdown voltage is unstable and a conductorwound inductor which is hard to fabricate so that the resultant magnetron is less expensive in production cost and much improved from the standpoint of fabrication efficiency. Further, the reduction of the number of parts due to the elimination of the capacitors and the inductors results in the reduction of the junction points so that the reliability of the junction points is much improved, and fewer parts need a smaller space and weigh less so that the resultant magnetron can be reduced in size and weight.

The microwave absorbing layer 24 shown in FIG. 3 is divided into three sublayers 24a, 24b and 24c but they may be integrally formed with the same material or may be formed separately with different kinds of materials. The sublayer 24a disposed between the inner conductors may be a ferrite plate or simply an ordinary insulating sheet. Various combinations of ferrite plate, ordinary insulating sheet, and rubber mixed with ferrite powder are also possible for the composite layer 24. In order to increase the effective frequency band width, it is perferable to form the sublayers 24a, 24b and 24c of different kinds of materials. The insulating layer 26 may be omitted in case where a very high withstand voltage is not required. The shape of the outer conductor is not limited to that described in this specification and shown in the attached drawings, either. Other modifications will be readily thought of by those skilled in the art, within the scope of the present invention.

What we claim is:

l. A magnetron comprising an anode and a cathode to define an operating space together with said anode; a plurality of cathode support bars to support said cathode; cathode leads connected with said plural cathode support bars and cathode lead-out wires connected with an external power source to supply electric power from said source for said cathode; a filter to eliminate noise signals connected between said cathode leads and said cathode lead-out wires; a stem to connect said cathode support bars with said cathode leads in vacuum condition; and a shield case to contain therein said filter and said stem, wherein said filter includes at least two microstrip type transmission lines having a configuration for choking at least one predetermined frequency component of an electromagnetic wave propagating therethrough and microwave attenuating substance covering said transmission lines for attenuating the wave over a wide frequency range.

2. A magnetron as claimed in claim 1, wherein said filter comprises said microstrip type transmission lines disposed near and substantially parallel to each other, serving as inner conductors; an outer conductor enveloping portions of said inner conductors; said microwave attenuating substance being filled between said inner conductors and between said inner conductors and said outer conductor.

3. A magnetron as claimed in claim 2, wherein each of said inner conductors has at least one projection whose length is such as to choke at least one predetermined frequency of the wave propagating through said conductor.

4. A magnetron as claimed in claim 3, wherein each of said inner conductors has a meander line inserted in a portion thereof surrounded by said microwave attenuating substance. I

5. A magnetron as claimed in claim 2, wherein said outer conductor is electrically connected with said shield case, and wherein the extension of one end of said inner conductors is drawn out of said shield case to serve as one of said cathode lead-out wires while the extension of the other end of said inner conductor is bent toward said stem to serve as one of said cathode leads.

6. A magnetron as claimed in claim 5, wherein each of said inner conductors has at least one pro ection whose length is such as to choke at least one predetermined frequency of the wave propagating through said conductor.

7. A magnetron as claimed in claim 6, wherein each of said inner conductors has a meander line inserted in a portion thereof surrounded by said microwave attenuating substance.

8. A magnetron as claimed in claim 2, wherein said microwave attenuating substance is one selected from among sintered ferrite and rubber containing ferrite.

9. A magnetron as claimed in claim 2, wherein an insulating layer is provided between said microwave attenuating substance and said outer conductor 10. A magnetron as claimed in claim 2, wherein each of said inner conductors has at least one looped portion whose length along the normal to the inner conductor is such as to choke at least one predetermined frequency of the wave propagating through said conduc- 

1. A magnetron comprising an anode and a cathode to define an operating space together with said anode; a plurality of cathode support bars to support said cathode; cathode leads connected with said plural cathode support bars and cathode lead-out wires connected with an external power source to supply electric power from said source for said cathode; a filter to eliminate noise signals connected between said cathode leads and said cathode lead-out wires; a stem to connect said cathode support bars with said cathode leads in vacuum condition; and a shield case to contain therein said filter and said stem, wherein said filter includes at least two microstrip type transmission lines having a configuration for choking at least one predetermined frequency component of an electromagnetic wave propagating therethrough and microwave attenuating substance covering said transmission lines for attenuating the wave over a wide frequency range.
 2. A magnetron as claimed in claim 1, wherein said filter comprises said microstrip type transmission lines disposed near and substantially parallel to each other, serving as inner conductors; an outer conductor enveloping portions of said inner conductors; said microwave attenuating substance being filled between said inner conductors and between said inner conductors and said outer conductor.
 3. A magnetron as claimed in claim 2, wherein each of said inner conductors has at least one projection whose length is such as to choke at least one predetermined frequency of the wave propagating through said conductor.
 4. A magnetron as claimed in claim 3, wherein each of said inner conductors has a meander line inserted in a portion thereof surrounded by said microwave attenuating substance.
 5. A magnetron as claimed in claim 2, wherein said outer conductor is electrically connected with said shIeld case, and wherein the extension of one end of said inner conductors is drawn out of said shield case to serve as one of said cathode lead-out wires while the extension of the other end of said inner conductor is bent toward said stem to serve as one of said cathode leads.
 6. A magnetron as claimed in claim 5, wherein each of said inner conductors has at least one projection whose length is such as to choke at least one predetermined frequency of the wave propagating through said conductor.
 7. A magnetron as claimed in claim 6, wherein each of said inner conductors has a meander line inserted in a portion thereof surrounded by said microwave attenuating substance.
 8. A magnetron as claimed in claim 2, wherein said microwave attenuating substance is one selected from among sintered ferrite and rubber containing ferrite.
 9. A magnetron as claimed in claim 2, wherein an insulating layer is provided between said microwave attenuating substance and said outer conductor.
 10. A magnetron as claimed in claim 2, wherein each of said inner conductors has at least one looped portion whose length along the normal to the inner conductor is such as to choke at least one predetermined frequency of the wave propagating through said conductor. 